Monolithic integrated capacitor

ABSTRACT

A monolithic integrated capacitor is formed by two conductive coatings applied to a substrate and separated from each other by a dielectric layer. The upper coating lying on the dielectric layer is connected via at least one conductive air bridge with at least one of a pair of connection lines of the capacitor. Parasitic inductances of the capacitor are largely compensated by connecting the two connection lines together by at least one high resistance line bridging the capacitor.

STATE OF THE ART

The present invention concerns a monolithic integrated capacitor, theelectrodes of which are formed by two conductive coatings applied to asubstrate and separated from each other by a dielectric layer, where theupper coating lying on the dielectric layer is connected with at leastone of the two connection lines of the capacitor via at least oneconductive air bridge. Such a monolithic integrated capacitor isdisclosed in “Design Guide, GaAs Foundry Services” by Texas Instruments,Version 4.2, February 1997, pages 1-6.

At very high frequencies e.g. in the millimetre wave range, monolithicintegrated capacitors do not have purely capacitative properties likeconcentrated capacitors, but under certain circumstances they also havea very high inductive reactance. This inductive part results mainly fromone or more conductive air bridges which connect a coating of thecapacitor with a connection line. The smaller the capacitor, i.e. thesmaller the dimensions of the coatings on the condenser, the greater thedistance to the connection lines and the longer the conductive airbridges must be made. However the longer the conductive air bridges, thegreater the parasitic inductive part of the reactance.

The invention is therefore based on the task of creating a monolithicintegrated capacitor, where the parasitic inductive part of thereactance is as small as possible.

ADVANTAGES OF THE INVENTION

The said task is solved by the features of claim 1 in that the twoconnection lines are connected together by at least one high resistanceline bridging the capacitor. This line constitutes an inductivereactance which is connected parallel to the other parasitic inductivereactances. The entire inductive part of the capacitor reactance is thussubstantially reduced.

According to the sub-claims, the monolithic integrated capacitor can beformed either as a series-connected element where the upper coating isconnected via a conductive air bridge to one of the two connection linesand the lower coating lying under the dielectric layer goes overdirectly into the other connection line. Or the monolithic integratedcapacitor can form a shunt to earth where the lower coating is broughtinto contact via a through contact in the substrate to an earth linepresent on the opposite substrate side and the upper coating isconnected with both connection lines via a conductive air bridge in bothcases.

DRAWING

The invention is now explained in more detail with reference to thedesign examples shown in the drawings. These show:

FIG. 1a a top view of a series-connected capacitor,

FIG. 1b a cross-section A—A of the capacitor shown in FIG. 1,

FIG. 1c a substitute circuit diagram of this capacitor,

FIG. 2a a top view of a capacitor forming a shunt to earth,

FIG. 2b a cross-section B—B of the capacitor shown in FIG. 2a, and

FIG. 2c a substitute circuit diagram of this capacitor.

DESCRIPTION OF DESIGN EXAMPLES

FIG. 1 a is a top view and FIG. 1b a cross-section A—A through amonolithic integrated capacitor on a substrate 1 (e.g. GaAS substrate).As the substitute circuit diagram shown in FIG. 1c of the monolithicintegrated capacitor shows, this is a series-connected capacitor C1.

The monolithic integrated capacitor consists in the known manner of alower coating 2 applied directly to the substrate 1 which forms thefirst electrode of the capacitor, a dielectric layer 3 applied to thisand an upper coating 4, applied to the dielectric layer 3, which formsthe second electrode of the capacitor. Also affixed to substrate 1 are afirst connection line 5 and a second connection line 6. In thecapacitor, formed here as a series-connected element, the lower coating2 goes over directly into the connection line 6. The second connectionline 5 is connected electrically conductively with the upper layer 4 viaa conductive air bridge 7. The conductive air bridge 7 is for exampledesigned as gold strips.

The conductive air bridge 7 causes a parasitic inductive reactance whichin the substitute circuit diagram in FIG. 1c is connected as aninductance L11 in series with the capacitor C1. This parasiticinductance L11 can be largely compensated by an inductance L1 connectedin parallel to the parasitic inductance L11 and capacitor C1. Such acompensating inductance L1, as shown in FIG. 1a, is created byconnecting the two connection lines 5 and 6 together by at least onehigh resistance line 8, 9 bridging the capacitor. Depending on the valueof the parasitic inductance to be compensated, under some circumstancesa single line 8 or 9 may suffice. For larger parasitic inductanceshowever it is recommended, as shown in the embodiment in FIG. 1a, toprovide two lines 8 and 9 bridging the capacitor. The size of theparasitic inductance L11 essentially depends on the length of theconductive air bridge 7. The length of the conductive air bridge 7 isgreater, the smaller the capacitor i.e. the smaller the upper coating 4.The smaller the upper coating 4, the greater the distance to be bridgedbetween it and the connection line 5. The entire inductive part of themonolithic integral capacitor becomes very low if the inductance L1 ofthe high resistance lines 8 and 9 is very much smaller than theparasitic capacitance¹ L11 of the air bridge 7.

1 German text uses “Kapazität” here although L11 should be an inductance(Inductivität).

FIG. 2a shows a top view, FIG. 2b a cross-section B—B and FIG. 2c asubstitute circuit diagram of a monolithic integrated capacitor whichforms a shunt to earth. The integrated capacitor has a lower coating 11applied to a substrate 10, over this a dielectric layer 12 and appliedto this an upper coating 13. The lower coating 11 and the upper coating13 form the two electrodes of the capacitor. The lower coating 11 isbrought into contact via a through contact 14 in the substrate 10 withan earth line 15 on the side of the substrate opposite the lower coating11. On the same substrate side on which coatings 11 and 13 of thecapacitor are applied, are two connection lines 16 and 17 which are eachconnected via a conductive air bridge 18 and 19 with the upper coating13 of the capacitor.

The two conductive air bridges 18 and 19, as shown in the substitutecircuit diagram in FIG. 2c, constitute two parasitic inductances L21 andL22. The through contact 14 also has an inductive effect which in thesubstitute circuit diagram is shown as the parasitic inductance L23connected in series with capacitor C2. The entire inductive reactance ofthe monolithic integrated capacitor can be very considerably reduced bymeans of an additional inductance L2 connected in parallel to the otherparasitic inductances. This additional inductance L2 is achieved bymeans of one or two high resistance lines 20, 21 which connect togetherthe two connection lines 16 and 17.

What is claimed is:
 1. A monolithic integrated capacitor, comprising: a)a substrate; b) a lower conductive coating applied to the substrate andforming one electrode of the capacitor; c) a dielectric layer applied tothe lower coating; d) an upper conductive coating applied to thedielectric layer and forming another electrode of the capacitor; e) apair of conductive connection lines applied to the substrate; f) atleast one conductive air bridge for connecting the upper coating withone of the connection lines; and g) at least one high resistance linebridging the capacitor and connecting the connection lines together. 2.The capacitor of claim 1, and further comprising another high resistanceline bridging the capacitor and connecting the connection linestogether.
 3. The capacitor of claim 1, wherein said at least one bridgeis a gold strip.
 4. The capacitor of claim 1, wherein the connectionlines are connected in series with the capacitor, and wherein the lowercoating merges directly into the other of the connection lines.
 5. Thecapacitor of claim 1, wherein said at least one bridge has a parasiticinductance in series with the capacitor, and wherein said at least onehigh resistance line has a compensating inductance in parallel with thecapacitor and compensating for the parasitic inductance.
 6. Thecapacitor of claim 1, and further comprising another conductive airbridge for connecting the upper coating with the other of the connectionlines.
 7. The capacitor of claim 6, and further comprising a conductivecontact for connecting the lower coating to ground.
 8. The capacitor ofclaim 7, wherein the substrate has one surface on which the coatings andconnection lines are applied, and an opposite surface having a groundplane; and wherein the contact extends between the surfaces of thesubstrate.
 9. The capacitor of claim 8, wherein the contact has acontact inductance in series with the capacitor and ground, and whereineach air bridge has a parasitic inductance; and wherein said at leastone high resistance line has a compensating inductance in parallel withthe parasitic inductances and compensating for the parasitic inductancesand the contact inductance.
 10. The capacitor of claim 9, and furthercomprising another high resistance line bridging the capacitor andconnecting the connection lines together.
 11. The capacitor of claim 1,wherein each connection line has a width, and wherein said at least onehigh resistance line has a width less than the width of a respectiveconnection line.